The Bangkok consulting and market-research firm said Thailand posted the region's strongest growth, with sales up 14.5 percent to 149,018 in the January-June period.
Sales rose 11.3 percent in Malaysia to 182,222 and 4.3 percent in Indonesia, to 132,057.
On the other hand, sales of new cars, trucks and buses in the Philippines dropped 14.8 percent to 35,379.
Thanks to its dominant, government-protected position in Malaysia, Perusahaan Otomobil Nasional Bhd., or Proton, was the volume leader in the four nations' combined markets, with a 20 percent market share.
Toyota Motor Corp. was next with a market share of 16 percent. It was followed by Mitsubishi Motors Corp. and Isuzu Motors Ltd. each with 11 percent; Perusahaan Otomobil Kedua Sdn. Bhd., or Perodua, with 10 percent; Nissan Motor Co. with 7 percent and Honda Motor Co. with 6 percent.